so like

function map(fn) {
  let result = { value, done }; // probably with those properties being non-configurable
  let underlying = this;
  return {
    next() {
      let { value, done } = underlying.next();
      if (!done) value = fn(value);
      result.value = value;
      result.done = done;
      return result;
    }
  }
}

Both? The iterator protocol imposes a lot of complexity; some of that complexity can be optimized through heroic work in JS engines (and has been!)... but the heroics mean that it's costly to do, particularly in any generalizable fashion.

I haven't looked at Iterator Helpers in a long while, but I'll bet they could certainly have more optimization applied over time, but I suspect a similar story applies: Could we make them faster? Sure, but that work displaces other work, and so we need to see it as important enough.